The present invention relates to a load sensing (LS)control system for supplying pressure medium to a hydraulic consumer.
LS control systems of this type are used in particular to control hydraulic consumers of mobile working devices. DE 197 15 020 A1 discloses an LS control block in which a consumer is supplied with a pressure medium via an LS directional control valve. The proportionally adjustable, directional control valve includes a valve spool that, together with a valve housing, forms a directional part and speed part. Via the directional part, the supply and release of pressure medium to and from the consumer is controlled, while the speed part establishes the volumetric flow rate of the pressure medium. In the case of the known solution, the speed part is formed by an adjustable metering orifice, one of which is located in the inflow and the other of which is located in the outflow, the opening cross-section of which may be changed via the axial displacement of the valve spool. A pressure compensator is installed upstream of the inlet metering orifice at the least, the pressure compensator being acted upon in the opening direction by the force of a spring, and by a pressure which exists downstream of the inlet metering orifice and corresponds to the load pressure, and, in the closing direction, is acted upon by the pressure that exists upstream of the inlet metering orifice.
If an LS control system of this type will now be used to control a recipricating cylinder using a compressive load, a load-lowering valve is often assigned thereto on the outlet side. A load-lowering valve of this type is basically a blocking valve that may be released via the pressure in the inlet, and which enables load to be lowered in a controlled manner in the presence of a compressive load. It has been demonstrated that a system of this type that includes an LS control block and a load-lowering valve tends to oscillate under certain operating conditions. This susceptibility to oscillation results from the fact that the load-lowering valve is controlled by the pressure in the inlet, i.e. by the pressure that exists downstream of the inlet metering orifice. If this inlet pressure is not sufficient, the load-lowering valve is closed and the return is closed. The pressure in the inlet then increases once more, and the load-lowering valve opens—the pressure in the inlet is therefore dependent on the extent to which the load-lowering valve is open. This opening and closing of the load-lowering valve that occurs when a load is being lowered results in fluctuations in the inlet that affect the upstream pressure compensator and, possibly, the variable-displacement pump, which is controlled as a function of the effective load pressure.
This tendency to oscillate may be reduced under certain circumstances by tapping the signaling pressure that acts on the pressure compensator between a pressure divider having a constant nozzle and a variable-area propelling nozzle, which is located in a control channel that extends between the inlet and the outlet. A solution of this type is disclosed, e.g., in EP 1 452 744 A1, which is owned by the applicant.
DE 38 02 672 A1 describes an LS control system, in the case of which the signaling pressure that acts on the pressure compensator in the opening direction is also tapped between a fixed nozzle and a variable-area propelling nozzle, both of which are integrated in a valve spool of an LS directional control valve and an LS control system. This control channel is connected to the tank via the variable-area propelling nozzle. The nozzle cross section is reduced depending on the displacement of the valve spool, and the tapped signaling pressure that acts on the pressure compensator in the opening direction increases accordingly, thereby preventing the pressure compensator from being closed—which would be undesirable—when a compressive load is lowered as described above. In the solution described in DE 38 02 672 A1, the signaling pressure is tapped via a further control channel in a conventional manner downstream of the inlet metering orifice when the valve spool is displaced in the opposite direction, e.g. to raise the load. The control channels and nozzles described above are integrated in the valve spool in a manner such that they are very difficult to manufacture in terms of forming the nozzle cross-sections the connecting channels.